Amplifier Input Bias Compensation Using Tail Current Recycling
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Solution Overview
Problem
Current amplifier designs face challenges in compensating for input bias currents without increasing power consumption or degrading noise performance and bandwidth.
Innovation Solution
A differential amplifier input stage with a compensation circuit that recycles the tail current through negative feedback, using a local amplifier to mirror the base current of the tail current source back to the input transistors, thereby reducing noise and maintaining bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If input bias current compensation is implemented using a secondary transistor mirroring base current, then input bias current compensation is achieved, but standing current and input current noise increase
Solution Approach 1:
The patent extracts the compensation function from the main signal path by using a separate secondary transistor circuit that mirrors only the base current component. This separates the compensation function from the signal amplification function, allowing compensation without adding noise to the signal path.
Solution Approach 2:
The patent creates a copy of the base current using a secondary transistor that mirrors the base current of the input transistors. This copied current is then fed back to compensate for the voltage drop caused by input bias current, achieving compensation without directly modifying the signal path.
2Measurement precision
If cascode transistors are used to buffer collector current and mirror base currents, then input bias current compensation is achieved, but bandwidth and positive common mode range are limited
Solution Approach 1:
The patent removes the cascode transistors from the signal path while maintaining their compensation function. By extracting the buffering function and keeping only the base current mirroring capability, the bandwidth limitation is eliminated while compensation is preserved.
Solution Approach 2:
The patent uses the secondary transistor as an intermediary element that indirectly provides compensation without being in the direct signal path. This mediator approach allows compensation to be achieved without the bandwidth limitations imposed by cascode transistors in the signal path.
3Measurement precision
If traditional input bias current compensation is implemented, then error voltage is reduced, but power consumption increases
Solution Approach 1:
The patent implements self-service by having the secondary transistor use its own base current (which is a byproduct of the input transistors' operation) to provide compensation. This recycled current approach minimizes additional power consumption while achieving error voltage reduction.
Data Source
AI summary
Low power low noise input bias current compensation for an amplifier input stage is provided by recycling the tail current of the differential pair transistors. A local amplifier regulates the tail current and buffers the base current of the tail current transistor, which is mirrored back to the input transistors to provide input bias current compensation.